Storage system



A. s. JENSEN 3,506,874

STORAGE SYSTEM 3 Sheets-Sheet 1 April 14, 1970 Filed Aug. l0, 196? IOOOV 800V April 14, 13970 A. s. JENSEN 3,506,874

STORAGE SYSTEM Filed Ag. 1o, 1967 S sheets-sheet 2 700V 5OO\/ 200V POTENTIAL SOURCE l |96 I f 2 OUTPUT '88a 2 r'go 20o POTENTIAL POTENTIAL POTENTIAL SOURCE SOURCE SOURCE Q I 202 l |98 1l;- J5

z POTENTIAL f f 2 SOURCE il: F|G.3. 3 C I |84 POTENTIAL SOURCE "3 PART 2O T h PART I PART1E- PARTE LL@ 2 m |5 a: \b ggg E IGP 1G 12 E :I D |2 3 C o E35 5 z (D b u u III I u O @j 2 b *S- J E m r 8 0 D-U m Z i.. 1 u I mz w b QZ LLI LLIuJ Q: 0 3 2Q zu: 4 E C SECONDARY om E25 To EMISSION RATIO Q52 m m (n 2 g2 2E o I I I I I I I I I I m g g 25 5o 75 IOO |25 |5O |75 2OO fr I-'g POTENTIAL OF TARGET SCREEN WITH E 4" RESPECT TO CATI-IODE ELEMENT o D lu I g -S-- E I.. E g 4 NVENTOR EL? |2" Arthur S. Jensen o mfg/M ATTORNEY April 14, 1970 A. s. JENSEN 3,506,374

` STORAGE SYSTEM Filed Aug. 10, l967 5 Sheets-Sheet I5 OUTPUT SIGNAL TO NOISE RATIO l I I I I I Y IO i 2'0 30 40 50 SO 70 8O INPUT VOLTAGE United States Patent O U.S. Cl. 315--12 18 Claims ABSTRACT F THE DISCLOSURE This invention relates to the method of operating and a system including a storage tube with a target and an electron gun therein. Illustratively the storage target may take the form of a layer of dielectric material and a tine screen overlying the dielectric layer. In another embodiment, the storage target may be an electrically conductive plate having a plurality of grooves therein and strips of dielectric material disposed within the grooves. In this embodiment the storage target appears on its surface to be a metal grille interleaved by dielectric strips. Writing is accomplished by scanning the target with an electron beam thereby to drive the surface of the dielectric material toward a first equilibrium potential as determined by a first potential source which is connected during the writing process to said screen (or grille). In order to derive an output signal, an electron beam is scanned across the storage target to drive the surface of the dielectric material towards a second, different equilibrium potential as determined by a second potential source which is connected during the reading process to said screen (or grille). IFurther, both of the potential sources are of a value above the first crossover of the secondary electron emission characteristic of the storage, dielectric material.

BACKGROUND OF' THE INVENTION Field of the invention This invention relates to electron discharge devices capable of storing a pattern of charges and more particularly to those systems including such electron discharge devices and appropriate potential sources for applying and retrieving patterns of charges.

Description of the prior art Typically, those electron discharge devices of the prior art which are capable of storing a pattern of charges (i.e. storage tubes) include an electron gun for directing a pencil beam of electrons onto a storage target. Such storage tubes known in the art as a Radechon typically include a barrier grid type storage target. Barrier grid storage targets normally include a metal plate upon which there is disposited a layer of storage dielectric material, and a screen or grid of a -iine mesh disposed upon the surface or adjacent to the layer of storage dielectric material. Such a layer of storage dielectric material is capable of storing a pattern of charges in response to the bombardment of electrons. More speciiically, a layer of storage dielectric material emits secondary electrons in response to electron bombardment thereby to drive the `surface of the storage dielectric material either positively or negatively depending upon the energy of the incident electrons. As is well known in the art, the ratio of the number of secondary electrons to the number of incident or primary electrons directed onto the layer of storage dielectric material emitted is dependent on the potential to which the bombarding, primary electrons are accelerated. Below a iirst value of accelerating potential typically known as iirst crossover, the ratio of secondary to primary electrons is less than one. As a result, the

3,506,874 Patented Apr. 14, 1970 bombarding electrons tend to charge the surface of the storage dielectric material negatively towards the potential of the source of electrons (i.e. cathode element). Between lirst crossover and a second value typically yknown as second crossover, the ratio of secondary to primary electrons is greater than one, typically reaching values as high as 6 or 7. A-s a result, the surface of the storage dielectric material is driven positively towards an equilibrium potential.

The equilibrium potential to which the surface of the dielectric material is driven depends upon the potential at which the grille or screen element is established. 'I'he secondary electrons emitted from the surface of the dielectric material in response to the bombardment of primary electrons do not all have the same level of energy i.e. the secondary electrons have varying velocities as they leave the surface of the storage dielectric material. Most investigators agree that the distribution per unit energy interval of the secondary electrons is approximately Maxwellian. In an illustrative example of the operation of a barrier grid storage target, the screen may be placed at a potential between first and second crossover with the result that the number of secondary electrons emitted from the storage dielectric material is greater than the number of incident primary electrons and that the net flow of electrons is away from the storage target thereby charging positively the surface of the dielectric material. As the surface of the storage dielectric layer is charged positively, a retarding electric iield is established between the layer of dielectric material and the screen such that slow secondary electrons are energetically unable to reach and pass through the screen. Since the screen is typically in contact with the layer of dielectric material, the slower secondary electrons are constrained to return to the layer of storage dielectric material within a mesh spacing from Where the secondary electrons were emitted. Thus, the screen prevents redistribution over the layer of dielectric material such as occurs within other storage tubes. Secondary electrons with greater energy are able to penetrate the screen and be accelerated to those electrodes disposed at a more positive potential. The charging process continues until the surface of the storage dielectric material reaches an equilibrium potential at which condition, the current of the fast secondary electrons which escape through the screen is equal to the current of the primary electrons incident upon the storage dielectric material. This equilibrium potential is somewhat positive with respect to the screen potential, in an amount which depends upon the secondary emision ratio and the average energy of the secondary electrons. (Reference: A. S. Jensen, Discharging an Insulator Surface by Secondary Emission Without Redistribution, RCA Review, vol. XVI, No. 22, pp., 216-233, June 1955.)

Typically, a pattern of charges may be disposed or written upon the layer of dielectric storage material by applying the input signal to the plate of the barrier grid storage target. Since the plate is disposed close to the surface of the storage dielectric material, a change in the potential applied to the plate capacitively changes the potential of the surface of the storage dielectric material so that it is no longer at equilibrium potential. In this method of operation, the screen is held at a constant potential and the control grid of the electron gun is also held at a constant potential to produce a constant density beam of primary electrons. As the surface of the layer of dielectric storage material is capacitively driven by the input video signal, a pattern of charges is established upon the layer of dielectric storage material due to the charging of the surface of the dielectric storage material towards the equilibrium potential as determined by the potential applied to the screen. This method is often called equilibrium Writing since it would be most effective if the charged areas were charged fully to equilibrium.

In another similar method, the potential of the screen is held constant While the potential source which is applied to the plate is such that it capacitively sets the potential of the surface of the storage dielectric material positive with respect to the equilibrium potential set by the screen. A video input signal is applied to the control element of the electron gun thereby 'to control the density of the primary electron beam as it is scanned across the surface of the layer of storage dielectric material. Again, the surface of the layer of storage dielectric material is charged toward the equilibrium potential determined by the potential of the screen and is charged an amount dependent upon the density of the electron beam and the dwell time of the -beam upon the layer of storage dielectric material. This method is often called non-equilibrium writing since to be effective no charged area may be permitted to charge entirely to equilibrium. (Reference: M. Knoll and B. Kazan, Storage Tubes and Their Basic Principles, pp. 22-27, Wiley 1952.)

Following the writing process, the retrieval or reading of the pattern of charges is accomplished by first removing the charging potential from the plate of the storage target element. Those portions of the target that were not written upon return to the equilibrium potential, whereas those portions that were written upon are negative with respect to the equilibrium potential by a potential difference dependent upon the amount to which these portions were discharged during the writing process. Typically, the beam current is held constant by applying a fixed voltage to the control grid of the electron gun. As the primary beam of electrons is scanned across the layer of storage dielectric material, those portions of the layer which are not at equilibrium potential are charged toward the equilibrium potential as determined by the potential applied to the screen. During the charging of these elemental portions of the storage target positively, a current is developed to the surface of the storage dielectric material which is capacitively coupled to the screen to provide the output signal. In an alternate method of obtaining an output signal corresponding to the pattern of charges stored upon the target, the return beam of secondary electrons may be used to provide an output signal. It is understood that the net secondary emission current from the storage target is dependent upon the voltage difference between the screen and the surface of the storage layer. Therefore, the return beam of electrons is dependent upon the pattern of charges as established during the writing process. Typically, the return beam of electrons may be picked up either by a wall or collector electrode or by an electron 4multiplier such as a series of dynodes to provide the output signal. (Reference: A. S. Jensen and G. W. Gray, Radechon Storage Tube Circuits, RCA Review, pp. 234-241, June 1955.)

In another storage tube typically known in the art as a Datachon or Gratechon, there is incorporated a grating storage target upon which a pattern of charges may be written and a repeated reading process may be performed by scanning the grating storage target with an electron beam without substantially discharging the pattern of charges stored thereon. (Reference: A. S. Jensen, W. G. Reininger and I. Limansky, The Grating Storage Target, Advances in electronics and Electron Physics, vol. 22A, pp. 155-173, Academic Press, 1966.) More specifically, the grating storage target includes a plate in which a plurality of grooves have been finely lined and partially filled with areas (i.e. strips) of a storage dielectric material so that portions of the plate and the dielectric material are disposed substantially within the same plane. The storage target appears as a grille interleaved by strips of dielectric. In operation, an electron gun including a cathode element for emitting a beam of electrons scans the beam of electrons onto the surface of the grating storage target. During the writing process, the plate 4 (i.e. the exposed metal grille) is held at a potential less than that of the first crossover of the dielectric material but positive with respect to the cathode element, so that the bombarded portions of the dielectric material are charged negatively and brought to an equilibrium potential with respect to the cathode element, that is, essentially to the same potential as the cathode element. During the reading process, the potential of the grille is adjusted so that the dielectric portions, which are capacitively coupled to the grille, are negative with respect to the potential of the cathode element whereas the grille is still positive with respect to the cathode element. Therefore, during the reading process, the scanning beam of electrons is in effect 4modulated by the negatively charged portions of the dielectric material as they fall upon the grilleV thereby to provide an output signal without erasing or attenuating the pattern of charges. This is known as non-destructive reading. In order to achieve destructive reading or fast erasing of the charges deposited upon the areas of dielectric storage material, the grille is disposed at a potential between first and second crossover and the grating storage target is scanned or bombarded with a beam of electrons. In a manner similar to that described above, the incident beam of electron drives the areas of dielectric storage material to an equilibrium potential dependent upon the potential applied to the grille. This operation is done with the grille disposed positively with respect to the other electrodes of the storage tube and efficient erasure requires that the voltage difference between the equilibrium potential and the grille potential be a constant.

The above described Radechon type storage tubes with a barrier grid storage target have been operated so that a constant voltage difference exists between the grid (or screen) and the equilibrium potential of the layer of storage dielectric material determined by the following physical constants of the material: the secondary emission ratio and the average energy of the secondary electrons. More specifically, during either the reading or writing processes, the potential of the surface of the layer of storage dielectric material is driven toward an equilibrium potential that remains constant with respect to the potential of the grille. This also means that the energy of the electron beam and the accelerating potential of the beam of electrons onto the surface of the layer of dielectric storage material has been so selected that the secondary emission ratio is substantially constant during the reading and writing processes because this ratio affects the potential difference between the screen and the equilibrium potential. Similarly, the destructive reading and rapid erasing of the grating storage target of the Datachon and Gratechon are conducted under conditions for which the potential difference between the equilibrium potential of the storage dielectric and the grille is a constant value.

As a result of this constant difference between the grille and equilibrium potentials, it is often necessary to apply potentials of significantly different values to the Various electrodes and in particular to the storage target in order to write and read the pattern of charges onto the storage target. Further, it may be necessary to apply video signals that are relatively large in voltage amplitude and also to sacrifice the dynamic range of the operation of such a device. Since in the Radechon these switching and video signals must be applied to the plate of the storage target and the plate has a high capacitance to the screen which must be held at a constant potential, these applied signals must be very powerful to drive the high capacitance through a sufliciently large Voltage difference. Further, as is the case of the grating storage target, it may be necessary to perform the writing process with potentials upon the storage target of less than the first crossover thereby allowing possible electron beam bending and sacrifice of the accuracy of placement of the electron beam onto the storage target. In addition, it is not now possible to apply bipolar video signals to storage tubes without the use for special biasing circuits for the various elements of these devices.

It is therefore, an object of the present invention to provide a new and improved method and system incorporating a storage discharge device in which the equilibrium SUMMARY OF THE INVENTION The present invention achieves the above-mentioned and additional objects and advantages by providing an improved system for storing information including an electron discharge device having therein a storage target, and an electron gun for directing a pencil beam of electrons thereon. The storage target includes a layer of dielectric, storage material and a grid or grille disposed upon or closely spaced from the exposed surface of the layer of the storage dielectric material to maintain the resolution of the stored pattern of charges. In order to accomplish the writing process, a suitable potential source accelerates a ow of electrons onto the dielectric material to dispose the surface of the layer of dielectric material at a first equilibrium potential. During the writing process, a second potential source accelerates a .beam of electrons onto the dielectric material to dispose the surface of the layer of dielectric storage material at a second equilibrium potential different from the first equilibrium potential. It is particularly noted that the first and second equilibrium potentials are chosen to be between the first and second cross-over potentials of the storage dielectric material.

In an illustrative embodiment of this invention, the storage target may take the form of a layer of dielectric material and a screen or grille disposed thereon. In an alternate embodiment of this invention, the storage target may include a plate member having a plurality of grooves disposed therein and areas or strips of dielectric material disposed on portions of the grooves Within the plate.

In accordance with one illustrative method of operating the storage system of this invention, an input video signal may be applied to the screen or grille of the storage target. The variableV amplitude of the input signal is applied to the grille or plate while an electron beam of substantially fixed current density is scanned or directed onto the surface of the storage target. As a result, the Iequilibrium potential of the surface of the storage dielectric material is thus varied by the input signal and the incident beam of electrons charges the surface toward the instantaneous equilibrium potentials. Alternatively, after an erasing step has been conducted with the grille disposed at the first potential, the grille is set at the second potential to establish the potential of the surface of the storage dielectric material at a potential different from the first equilibrium potential, and the variable input signal may be applied to the control element of the electron gun. In a similar manner, a photocathode could be used in place of the electron gun to write a pattern of charges onto the storage target.

In order to read out the pattern of charges, the potential applied to the screen or grille of the storage target is changed from that of the Writing process. More specifically, a new equilibrium potential is established by applying a second potential of a value different from that of the first potential and, during the reading process, charging the surface of the storage dielectric material toward a new equilibrium voltage dependent upon the second potential.

6 BRIEF DESCRIPTION OF THE DRAWINGS These and other objects and advantages of the present invention will become more apparent by referring to the following detailed description and the accompanying drawings, in which:

FIGURE 1 is a diagrammatic representation of a system including an electron discharge device for information storage and suitable potential sources for operating the electron discharge device in accordance with the teachings of this invention;

FIGS. 2 and 3 are diagrammatic representations for alternative systems and electron discharge devices to be operated in accordance with the teachings of this invention;

FIG. 4 is a graphical representation of the Ivoltage difference between the grille of the storage target and equilibrium potential of the surface of the storage dielectric material as a function of the electron accelerating potential applied to the grille of the storage target to be incorporated in the system of this invention; and

FIG. 5 is a graphical representation demonstrating the signal to noise ratio of systems in accordance with the teachings of this invention as compared with storage systems of the prior art.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings and in particular to FIG. 1, there is shown a storage system 10 including a storage tube 11 having an evacuated envelope 12 and an electron gun 14 disposed at one end of the envelope 12 and a storage target 44 disposed at the other end. The electron gun 14 includes the usual indirectly heated cathode element 16 surrounded by a control element or grid 18. Adjacent to the control element 18, there is disposed an anode element 20, the end of which constitutes the first dynode of an electron multiplier. As shown in FIG. l, the cathode element 16, the control element 18 and the anode element 20 have apertures centrally aligned therein through which a beam 40 of electrons is projected towards the storage target 44. A tubular electrode 22 typically known as a persuader is axially placed with respect to the electron gun 14 and serves to direct the secondary electrons emitted from the first dynode 20 into the succeeding electron multiplier dynodes. Further, a wall coating 24 is applied to the interior surface of the envelope 12 in order to establish the drift energy of the electron beam 40 en route to the storage target 44. Deiiecting coils 26 are disposed about the envelope 12 in order to deflect the electron beam 40 in a vertical and horizontal direction. It is understood that these coils are impressed with a varying voltage such as a sawtooth potential to produce a line and page scanning raster. Further, a focusing coil 28, which produces a magnetic field parallel to the axis of the tube, is disposed about the envelope 12 in order to focus the beam 40 of electrons onto the storage target 44.

About the electron gun 14, there is placed a plurality of multiplier dynodes 30, 32 and 34 and a collecting electrode 38. As is well known in the art, the electron multiplier dynodes 30, 32 and 34 have openings fitted about the electron gun and further have a plurality of blades 36 which extend radially from the control element 18. The blades 36 may be coated with any desired electron emissive material thereby to multiply successively or increase the number of electrons of the return beam 42 of electrons which is directed first upon the anode' element 20 and then successively applied to the electron multiplier dynodes 30, 32 and 34 to be finally collected by the collector 38 to provide an output signal.

The storage target 44 illustratively comprises a layer 46 of a suitable storage dielectric material such as laluminum oxide or magnesium fluoride, and a grille or screen 48 disposed to overlie the surface of the layer 46 exposed to the electron gun 14. The screen 48 may be made of a suitable electrically conductive material such ias copper or nickel and have a mesh size of 1500 openings per inch and 50% open area. The screen 48 is preferably disposed in contact with the layer 46. As the screen 48 is spaced from the surface of the layer 46 of storage dielectric material, the secondary electrons emitted from the layer 46 tend to spread out past the screen 48 and to redistribute over the surface of the storage layer 46 thereby destroying the resolution of the pattern of charges stored on the target 44. In order to obtain a resolution of a few lines per mm., the screen 48 should not be spaced from the surface of the storage layer 46 by more than 0.02 mm.

For a more detailed description of the storage tube as described above, reference is made to U.S. Patent No. 2,548,405.

As shown in FIG. 1, the cathode element 16 is connected to ground, and the control element 18 is connected by a switch 58 to either of the potential sources 60 and 62. Further, the screen 48 of the storage target 44 is connected through a resistive impedance 70 and a potential source 68 to ground. In addition, the screen 48 may also be connected through a switch 64 to a video signal input potenti-al source 66 to ground. The collector electrode 38 of the electron multiplier is connected through a resistive impedance 56 and a potential source 54 to ground. Further, the collector electrode 38 is connected by a switch 50 to an amplifier 52 to provide an output signal from the storage system 10.

In order to appreciate more fully the contribution of this invention, reference is made to FIG. 4 which presents in graphical form measurements of the voltage difference between the screen 48 and the equilibrium voltage of the surface of the layer 46 of storage dielectric material as a function of the potential applied to the screen 48. For screen potentials less than that of the first crossover of the storage dielectric material of the layer 46, the surface of the layer 46 reaches as an equilibrium the potential applied to the cathode element 16, as shown on the left-hand portion of the graph of FIG. 4. For screen potentials above first crossover, the surface of the layer 46 of storage dielectric material cornes to an equilibrium potential with respect to the potential applied to the screen 48, as shown on the right-hand side of the graph of FIG. 4. The equilibrium potential curve may be divided into four parts corresponding to the following sections of the secondary electron emission curve of the layer also shown on FIG. 4:

(I) That section of the curve in which the secondary emission ratio of the dielectric layer increases with screen potential;

(II) That section where the secondary emission ratio of the dielectric storage material is approximately constant; and

(III) That section of the curve where the potential of the grille is not substantially negative to the potential of the surrounding electrodes (particularly the wall 24 and the first dynode 20, referring to the embodiment of FIGURE 1);

(1V) That section of the curve where the potential of the grille is more positive than that of the surrounding electrodes.

In accordance with the teachings of this invention, the reading, writing and erasing processes should take place where the equilibrium potential curve has a substantial slope. Further, the writing and reading processes are most efficient where the slope is the steepest. Therefore, as shown in FIG. 4, the storage system of this invention would operate inefiiciently if the reading and writing processes were performed on those portions of the curve where the slope is the smallest; illustratively, in Part II of the curve where the potential is in the order of about 115 volts. Further, it is undesirable to operate a storage target with the grille potential so near to the potential applied to the wall coating 24 that the surface of the 8 layer 46 becomes positive with respect to the wall coating 24, i.e. the Part IV of the curve. If this condition occurs, secondary electrons from the layer 24 will redistribute over other portions of the layer 24, thus reducing signal resolution.

Generally, the prior art performs the operations of (a) writing, (b) reading, and (c) erasing by discharging the surface of the layer of storage dielectric material towards the saine equilibrium potential and at the same point on the secondary emission ratio curve. The distinctiori between writing and erasing in the prior art, for example in the Radechon storage tube, is made by setting the potential of the storage dielectric surface respectively above or below the equilibrium potential. This is accomplished by changing the potential of a plate, which is capacitively coupled to the storage dielectric surface, with respect to the potential of the screen, which is usually disposed at a constant potential. As shown in FIG. 4, writing (process a) upon the layer 46 of storage dielectric material may be performed by discharging the surface of the storage dielectric layer negatively toward a given point on the equilibrium potential curve. In order to read the information of pattern of charges from the storage target, the surface of the dielectric layer is driven negatively with respect to the screen by applying an appropriate potential source to the plate which is capaci tively coupled to the surface of the storage layer then, a beam of electrons is accelerated with approximately the same energy (which is determined by the screen potential) as that of the writing electron beam onto the layer of dielectric material of the storage target thereby to discharge the potential of the surface of the layer of storage dielectric material positively towards the equilibrium potential (process (b)). In other words the writing process (a) and the reading process (b) take place at the same point on the equilibrium potential and the secondary emission curves. In a similar manner, the erasing process (c) is accomplished by further discharging the potential of the surface of the storage dielectric material towards the equilibrium potential at the same point on the secondary emission curve.

The operation of the storage system in accordance with the teachings of this invention takes place in three steps: (a) writing, (b) reading and (c) erasing.

Writing-With particular regard to the embodiment of the invention shown in FIG. 1, the writing process takes place with the switch 64 in its first position and the video input source 66 connected across the resistive impedance 70. Further, the switch 58 is connected in its first posititon in order to connect the potential source 60 to the control element 18. The switch 50 is disposed in its second position in order that no signal be applied to the output. The electron gun is biased close to zero and the source 60 may have an illustrative value of approximately 1 or 2 volts in order that a suitable constant current beam 40 of electrons may be scanned across the surface of the storage target 44. Depending upon the magnitude of the potential of source 68, the writing will take place on either the positive (Part I) or negative (Part III) portions of the curve shown in FIG. 4. In a first method of operation, the potential 'source 68 could be set at a value of approximately 30 volts and the video input 66 could apply a single amplitude signal of approximately 5 to l0 volts across the resistive impedance 70 to the screen 48. As seen in FIG. 4, the amplitude of the input signal applied to the screen 48 determines the point on the secondary emission ratio curve at which the storage target 44 will operate and also the equilibrium potential toward which the surface of the dielectric layer 46 will be discharged. The first method is illustratively shown to be operating on the positive portion of the equilibrium graph of FIG. 4. As lthe equilibrium potential of the surface of the dielectric layer 46 is varied by the input signal applied to the screen 48, the electron beam 40 charges the surface of the layer 46 towards the corresponding instantaneous equilibrium potentials on the curve of FIG. 4. As seen in FIG. 4, the greater the amplitude of the input signal, the more the surface of the layer 46 will tend to be charged, but always toward the equilibrium potential. Illustratively, writing is performed by charging the dielectric layer only 50% of the difference Ibetween the screen potential and the equilibrium potential. This degree of charging is thought to be a reasonable compromise between operating with a large beam current to achieve poor resolution but with a high writing data rate, and alternatively, to achieve high resolutiton with a low writing data rate.

In an alternative method of operation, a bipolar input signal with a double amplitude between to 20 volts peak-to-peak may be applied to the screen 48. As shown on the negative slope portion of the graph of FIG. 4, the potential source 68 could be set at approximately 150 volts with the negative and positive sweeps of the input signal to the screen 48 capacitively driving the surface of the layer 46 so that the instantaneous equilibrium potential is thereby changed. The constant current electron beam 40 then discharges the storage dielectric surface respectively positively and negatively toward the instantaneous equilibrium potentials of the curve. In both methods of operation, writing is accomplished at a potential between the first and second crossover potentials of the storage dielectric material.

Reading-In order to read or obtain a signal corresponding to the pattern of charges placed upon the surface of the layer 46 of dielectric storage material by the first alternative process described above, the switch 64 is disposed in its second position, the switch 58 remains disposed in its first position thereby connecting the control element 18 to the potential source 60, and the switch 50 is disposed in its first position thereby connecting the collector electrode 38 through the amplifier 52 to the output of the system 10. With a constant potential applied by the potential source 60 to the control elernent 18, the constant current electron beam 40 is scanned across the surface of the dielectric layer 46. The electron beam 40 is accelerated by the potential applied by the source 68 to the screen 48. The potential source 68 is typically chosen to be the same as was used during the writing process (a), but the absence of the video input signal has the result that the reading process (b) is accomplished at a potential between the first and second crossover potentials of the storage dielectric material. As shown in FIG. 4, the reading process (b) is disposed on the positive portion (Part I) of the curve at a new point on the secondary emissive curve dilierent from its instantaneous value during the writing process (a), and the surface of the layer 46 is driven negatively toward equilibrium. In order to provide an output signal, the secondary electrons emitted from the surface of the storage dielectric layer 46 are focused as by the coil 28 onto the anode element 20. As explained above, the electrons are directed by the persuader electrode 22 onto dynode 30, and successively therefrom to dynodes 32 and 34, and are collected by the collector electrode 38 to provide a video potential variation across the resistive impedance 56. With the switch 50 disposed in its first position, the potential developed across the impedance 56 is amplified to provide a suitable output signal. It is a significant aspect of this invention that the reading process takes place at a different point upon the secondary emission curve and that, therefore, the potential of the surface of the layer of dielectric storage material 46 is driven toward an equilibrium potential different than that to-ward which the writing process(es) (a) occur. As shown on the negative slope (Part III) portion of the equilibrium potential curve of FIG. 4, the reading process of the second or alternative method takes place at a single potential and those portions, which are charged negatively and positively, are driven respectively positively and negatively toward equilibrium potential at the same point on the curve, whereas the writing process (a) took place at a series of points on the curve different from that of the reading process.

Erasing-It is noted that the reading process(es) (b) described above partially discharges the charge pattern disposed upon the layer 46. However, it is not practical to discharge the entire pattern of charges with a single sweep of the electron beam 40. Therefore, it is often desirable to provide a separate erasing process (c) in Iwhich the beam 40 is repeatedly scanned across the surface of the layer 46 of the dielecnic storage material thereby to erase substantially the pattern of charges from the storage target 44. In this process, the switch 58 is disposed in its second position thereby to connect the potential source 62 to the control element 18, and the switch 50 is disposed in its second position. The source 62 is preferably of a greater potential than that of source 60 to thereby provide a beam 40' of greater current though, doubtless, of lower resolution. 'Ihe beam 40 of greater current density is then scanned successively two or three times across the surface of the layer 46 thereby to erase substantially the pattern of charges.

Referring now to FIG. 2, there 1s shown a storage system including a storage tube 84 having an evacuated envelope l82 and an electron gun 83 disposed at one end of the envelope 82. The electron gun 83 generates and scans an electron beam 86 over the surface of a storage target -88 disposed at the other end of the envelope 82. The electron gun y83 includes a cathode element 89, which may be a short tubular member having a closed face with a layer of electron emissive material deposited thereon, and a control element 90 having an aperture through which the electrons are accelerated. A first accelerating electrode 92, a decelerating plate electrode 94, and a second accelerating plate electrode 96 are disposed along the axis of the envelope 82 in a spaced relation with each other. Each of the electrodes 92, 94 and 96 have apertures therethrough disposed about the axis of the envelope 82 to provide passage for the beam `86 of electrons. The potential difference between the electrode 94 and the electrodes 92 and 96 forms an electron lens to focus the electron beam 86 onto the storage target 88 in a manner well known in the art. The beam 86 of electron passes between a pair of horizontal deflecting plates 98 and a second pair of vertical deecting plates 100 to which appropriate potential sources (not shown) are connected to cause the beam 86 of electrons to be scanned in a raster over the surface of the storage target 88. A shield electrode 102 is disposed between the pairs of deliecting plates 98 and 100. In addition, a tubular electrode 104 is disposed between the electron gun 83 and the storage target 88 for collimating the beam 86 of electrons to impinge perpendicularly onto the storage target 88. Further, an annular electrode 112 may be disposed between the electron gun -83 and the collimating electrode 104 to collect the secondary emitted electrons from the surface of the storage target 88 and to provide an output signal therefrom.

The storage target 88 illustratively comprises a layer 108 of a suitable dielectric storage material such as aluminum oxide, magnesium fluoride or mica, and a plate support electrode 106 made of a suitable electrically conductive material. As shown in FIG. 2, the plate electrode 106 is disposed on the side of the layer 108 remote from the electron gun 83. Further, a grille or screen is disposed in contact with or closely spaced from the exposed surface of the layer 108 of storage dielectric material. In accordance with the teachings of this invention, the plate electrode 106 is not necessary for the operation of the storage system and may either be deleted or as shown in FIG. 2, connected electrically to the screen 110. For a more complete description of the storage tube as shown and described with respect to FIG. 2, reference is made to U.S. Patent No. 2,770,747 by the inventor of this in- 1 1 vention and also to (The Radechon, a Barrier Grid Storage Tube, by A. S. Jensen, RCA Review, pp. 197- 215, June 1955).

Appropriate potentials are applied to the various elec trodes of the storage device as shown in FIG. 2 in order to operate the system 80 as will be explained. Specifically, a switch 126 may be disposed in first and second positions to respectively connect a potential source 130 or a potential source 128 to the screen 110 of the storage electrode 88. When the switch 126 is disposed in its second position, the screen 110 is connected through a resistive impedance 132 and the potential source 128 to ground. Further, a source 116 of a video input signal is connected by a switch 114 to the control element 90. The switch 114 may be disposed in its second position thereby to connect a potential source 118 to the control element 90. Illustratively, the cathode element 89 may be connected to ground and an appropriate voltage drop may be sustained between thecontrol element 90 and the cathode element 89 by a resistive impedance 120 disposed therebetween. In an alternative method of operating this invention, an output signal may be obtained through the collector electrode 112 which may be connected through a resistive impedance 122 and a potential source 127 to ground.

In order to write or to place a pattern of charges upon the surface of the storage layer 108, the switch 126 will be disposed in its iirst position to connect the potential source 130 to the screen 110. The switch 114 is disposed in its first position to connect the video input source 116 to the control element 90 across the resistive impedance 120. The input signal from the source 116 varies the current density of the beam 86 of electrons as it is scanned by the electron gun 83 over the surface of the layer 108. As a result, the modulated beam 86 of electrons thus charges the surface of the layer 110 towards a single point on the equilibrium curve dependent upon the potential of the source 130 by amounts proportional to the beam current and the dwell time.

In order to obtain an output signal or to read the pattern of charges, the switch 126 is disposed in its second position thereby to connect the screen 110 through the resistive impedance 132 and the potential source 128 to ground. The switch 114 is disposed in its second position in order to connect the DC potential source 118 to the control element 90 to provide a constant current beam 86 of electrons. The potential source 128 provides a potential of approximately 5 to 10 volts more negative than that applied 'by the source 130 to the screen 110. Therefore, the beam 86 of constant current density drives the surface of the storage layer 108 towards a new point on the equilibrium curve as determined by the potential applied by the source 130. In order to discharge the surface of the layer 108, a current is drawn through the screen 110 and the resistive impedance 132 to provide an output signal corresponding to the pattern of charges deposited upon the storage target 88. In an alternative method of obtaining an output signal, the varying secondary emission of electrons as the surface of the layer 108 is driven toward a new equilibrium potential provides a varying current of secondary emitted electrons which may be collected by the electrode 112 to provide an output signal across the resistive impedance 122. As mentioned above, the reading process partially discharges the surface of the layer 108 and it is normally necessary to scan the target 88 several more times in order to erase or discharge substantially the pattern of charges stored thereon. Referring to FIG. 4 this method is performed on the positive (Part I) portion of the curve wherein the writing process (a) is done at the higher potential, and the reading process (b) is done at the lower potential for which the equilibrium potential is different.

Referring now to FIG, 3, there is shown an alternative embodiment of this invention which takes the form of a storage system 140 including a storage tube 141 having an evacuated envelope 142. An electron gun 144 is disposed within one end of the evacuated envelope 142 for directing a beam of electrons onto a storage target 162. The electron gun 144 includes a cathode element 146 for generating electrons, a control element or grid 148, and an anode element 150. The elements 148 and 150 have apertures disposed about the axis of the envelope 142 through which the beam of electrons is accelerated. The anode element 150 also serves as the rst 'dynode for an electron multiplier. A wall electrode 154 is disposed upon the interior of the envelope 142 to determine the `drift energy of the electron beam. A persuader electrode 152 is disposed between the anode element 150 and the wall electrode 154 for directing into the electron multiplier the secondary electrons emitted by the anode-first dynode 150. Further, deflection coils 158 are disposed about the exterior of the envelope 142 between the electron gun 144 and the storage target 162 to deflect the beam of electrons emitted by the electron gun 144 in a desired pattern or raster across the surface of the storage target 162. Further, a focusing coil 156 provides a suitable axial, magnetic eld for focusing the beam of electrons onto the storage target 162.

An output signal may be derived by collecting the secondary electrons emitted from the storage target 162. The secondary electrons emitted by the iirst dynode 150 in response to the return beam from the storage target 162 are directed by the persuader electrode 152 onto a second dynode 204 of the electron multiplier. From the first dynode element 150, the electrons are accelerated into the electron yrnulitplier which includes a plurality of electron dynodes 204, 206 and 208 which act successively to increase and multiply the number of electrons. The electrons are collected by an electrode 210 to provide an output signal.

The storage tube as shown in FIG. 3 is capable of sensing radiation images emanating from a scene 172. More specifically, a radiation image is focused by a suitable lens assembly 174 onto a photocathode element 170 which, in turn, emits a corresponding electron image. A suitable spiralled, accelerator electrode 176 is disposed upon the interior surface of the enlarged portion of the envelope 142 to accelerate the electrons emitted by the photocathode element onto the storage target 162. Further, a coil 178 is disposed about the envelope 142 to provide an axial magnetic eld to focus the photoelectron image onto the storage target 162 in a manner well known in the art. The storage target 162 is pivotally mounted within the envelope 142 upon a pair of pins 180. There is provided a support structure (not shown) within the envelope 142 upon which the pins 180 are mounted and which allows the target 162 to be pivoted. In one illustrative embodiment, an olset mass 182 is disposed at one end of the target 162 so that by tilting the envelope 142, the target 162 may be rotated due to the gravitational forces acting upon the offset mass 182, Alternatively, a coil of wire (not shown) may be wound circumferentially around the storage target 162 to provide an electric eld which reacts electrodynamically with the axial magnetic focusing eld to rotate the target 162. A detent spring (not shown) may be used to hold the storage target in each of two rest positions.

The storage target 162 illustratively includes an electrically conductive back plate 164 having a plurality of grooves 166 machined therein. A plurality of strips or areas 168 of storage dielectric `material are disposed along one sideof the grooves 166 as shown in FIG. 3. For a more complete description of the storage target 162 and the method of prepanation of the target, reference is made to U.S. Patent No. 3,218,496 of the inventor of this in vention and assigned to the assignee of this invention.

As shown in FIG. 3, the photocathode element 170 is connected to ground. Further, a switch 192 may be disposed in its rst position to connect a potential source 188 to the back plate 164 of the storage target 162 and in a second position to connect a potential source 190 to the back plate 164. An output signal may be obtained by disposing a switch 196 in its first position to connect the collector electrode 210 through a resistive impedance 195 and a potential source 194 to ground. As shown in FIG. 3, the cathode element 146 isconnected to ground. The control element 148 is connected by a switch 202 disposed in its first position to a potential source 200, in its second position to a second potential source 198, and in its third position a third potential source 184.

In order to write or place a pattern of charges upon the areas 168 of the storage dielectric material, the switch 192 is disposed at its lirst position to connect `the potential source 188 to the back plate 164 of the storage target 162, and the switch 202 is disposed in its first position to connect the potential source 200 to the control element 148. The potential source 200 applies a constant potential to the control element 148 so that the electron beam is cut off. Further, the switch 196 is disposed in its second position to disconnect the collector electrode 210 from the output. It may be understood that in order to write a pattern of charges, the target 162 is rotated from the position shown in FIG. 3 so that the areas 168 of storage dielectric material face the photocathode element 170. The photocathode element 170 projects a photo electron image in response to the radiation image directed thereon which is accelerated as by the electrode 176 and focused by the coil 178 onto the storage target 162. The incident electrons drive the various areas 168 of the storage dielectric toward an equilibrium potential as determined by the potential of the source 188 which is applied to the back plate 164. Illustratively, the writing process (a) may be conducted on the negative slope (Part III) of the equilibrium curve shown in FIG, 4.

In order to read or derive a signal corresponding to the pattern of charges disposed upon the storage target 162, the storage target 162 is rotated so that the areas 168 of storage dielectric material are disposed towards the electron gun 144. The switch 192 is disposed in its second position to connect the potential source 190 to the back plate 164 of the storage target 162. In addition, the switch 202 is disposed in its second position to connect the potential source 198 to the control element 148, and the switch 196 is placed in its first position to provide an output signal from the collector electrode 210. The source 198 applies a DC potential between the cathode element 146 and the control element 148 to thereby provide a constant current beam of electrons which is scanned across the surface of the storage target 162. In response to the incident beam of electrons, the surface of the areas (or strips) 168 of storage dielectric material are driven toward a new equilibrium potential as determined by the potential source 190, which is illustratively 5 to 2() volts either positive or negative with respect to the potential of source 188. Referring again to FIG. 4 the reading is in accord with one o f the processes (b) in the diagram on the negative slope (Part III) of the curve. As the various portions of the areas 68 of storage dielectric material are driven toward equilibrium potential, the secondary emission of electrons is varied. As a result, the return beam of secondary electrons emitted from the storage target 162 is proportional to the pattern of charges disposed upon the storage target 162. The return beam of secondary emitted electrons is focused by the wall electrode 154 onto the anode element 150. In turn, the secondary electrons emitted by the anode element 150 are directed by the persuader electrode 152 to the dyuode elements 204, 206 and 208 to be collected `by the electrode 210 to develop an output signal across the resistive impedance 195. It is particularly pointed out that the reading and writing operations as described with respect to the storage tube of FIG. 3, depends upon driving the areas or strips 68 of storage dielectric material toward two different equilibrium potentials.

In order to complete the erasing of the pattern of charges disposed upon the storage target 162, the switch 202 is disposed in its third position to connect the potential source 184 to the control element 148. rIypically, the potential of source 184 is a more positive than the source 198 thereby to provide an electron beam of greater density to achieve a more rapid erasure of the pattern of charges still remaining upon the storage target 162.

Referring now to FIG. 5, there are shown curves representing the signal to noise ratio as a function of the input voltage for the various processes of operating the storage tube as shown in FIG. 1 in accordance with the teachings of this invention and in accordance with the processes of the prior art. More specifically, the input voltage may refer either to the peak-to-peak voltage of the input video signal which is applied to the screen of the storage target or to the difference between the potentials applied to the screen during the reading and writing processes. Curve A represents the signal to noise ratio achieved by applying the input video signal to the screen of the storage target (as shown in FIG. 1) and Writing by driving the surface of the dielectric storage toward an equilibrium potential in accordance with the teachings of this invention. Curve C represents the signal to noise ratio achieved by placing the input signal to the back plate of a storage target in accordance with a typical method of operating a device of the prior art such as a Radechon tube. It is noted for those portions of the curves above approximately 30 volts that the signal to noise ratios achieved by the teachings of this invention are higher than those of the prior art. Curve\ B represents the signal to noise ratio achieved by driving the surface of the storage dielectric material toward the cathode potential to read out the stored pattern of charges. Only those portions of the curve B above 55 volts exceed the results of curve A and further the low energy writing process of curve B does not function below 35 volts (Le. tirst crossover of the storage dielectric material).

A significant advantage of the system and method of operation of this invention lies in the elimination of the back plate of the prior art storage target. Typically, the input signal, in accordance with the teachings of the prior art, is applied to the back plate of the storage target thereby to drive capacitively the surface of the dielectric storage material. The capacitance presented to the input or switching potentials applied to the back plate is in the order of 1000 to 5000 picofarad. In accordance with the teachings of this invention, the input or switching signals are applied to the screen of' the storage target which presents a capacitance of only 10 to 20 picofarad. This higher impedance requires a much less powerful video driver.

Further, the reading and writing processes in accordance with the teachings of this invention are performed quickly in that the discharge factor is high during these processes. Further, the switching potentials applied to the storage target and to the other electrodes of this device are of a relatively mall magnitude and typically less than volts. Further, the amplitude of the input video potential as applied either to the screen of the storage target or to the control element of the electron gun is relatively small thereby requiring less driving power. In addition, the writing and reading processes of this invention achieve a larger dynamic range than that achieved @by the methods and systems of the prior art, and the use of a high energy electron beam for reading has an advantage of negligible beam bending, hence better positional accuracy. Further, a bipolar video signal may be applied to the grille of the storage target without the need for special biasing circuits as required with the systems of the prior art.

Since numerous changes may be made in the abovedescribed apparatus and different embodiments of the invention may be made without departing from the spirit thereof, it is intended that all matter contained in the foregoing description and shown in the accompanying drawings, shall be interpreted as illustrative and not in a limiting sense.

I claim as my invention:

1. A storage electron tube system comprising a storage target including areas of storage dielectric material and of electrically conductive material disposed upon a. common surface; first means for writing with a first ow of electrons a charge pattern on said areas of storage dielectric material, said first means including a first potential source of a magnitude for accelerating said first fiow of electrons onto said areas of storage dielectric material so as to establish a first equilibrium potential for the surfaces of said areas of storage dielectric material; and second ymeans for reading said charge pattern with a second ow of electrons, said second means including a second potential source of a magnitude differing from that of said first potential source for accelerating said second flow of electrons onto said areas of storage dielectric material so as to establish a second equilibrium potential for the surfaces of said areas of storage dielectric material, the magnitudes of said first and second sources selected to be between the first and second crossover potentials of said storage dielectric material.

2. A storage electron tube system as claimed in claim 1, wherein said rst potential source provides means for applying an input signal to control said charge pattern.

3. A storage electron tube system as claimed in claim 1, wherein said second potential source provides means for deriving an output signal.

4. A storage electron tube system as claimed in claim 1, wherein there is included means for collecting a return beam of electrons from the surfaces of said areas of storage dielectric material thereby to derive an output signal. v

5. A storage electron tube system as claimed in claim 4, wherein said means for collecting includes an annular electrode, and a third potential source for providing an accelerating potential to said annular electrode.

`6. A storage electron tube system as claimed in claim 4, wherein said means for collecting includes a plurality of dynodes for successively multiplying the electrons of said return beam and a collector electrode for collecting the multiplied beam of electrons thereby to provide an output signal.

7. A storage electron tube system as claimed in claim 1, wherein said storage target takes the form of a layer of said storage dielectric material and of an electrically conductive mesh overlying the surface of said layer.

`8. A storage electr on tube system as claimed in claim 7, wherein said mesh is disposed in contact with said layer.

9. A storage electron tube system as claimed in claim 1, wherein said first means includes a source for said first flow of electrons having at least a cathode element for generating electrons and a control element, and a source of input information which is electrically connected to said control element.

10. A storage electron tube system as claimed in claim 1, wherein said first means includes a source for said first flow of electrons having a photocathode element for providing an electron image in response to a radiation image. v

11. A storage electron tu'be system as claimed in claim 10, wherein said second means includes an electron gun for providing said second ow of electrons, said storage target is capable of being positioned in either a rst position so that said areas of storage dielectric material are exposed to said first flow of electrons or a second position wherein said areas of storage dielectric material are exposed to said second flow of electrons.

12. A storage electron tube system as claimed in claim 1, wherein said storage target takes the form of a plate having a plurality of grooves disposed therein, and said storage dielectric material is disposed upon the sides of said grooves.

13. A method of operating a storage tube having a target including areas of an electrically conductive material and of a storage dielectric material disposed upon a common surface, said method comprising the steps of placing a charge pattern upon said target -by directing a first flow of electrons onto said areas of storage dielectric material and accelerating said first iiow of electrons to first potential to establish a first equilibrium potential for the surfaces of said areas of storage dielectric material, and deriving an output signal corresponding to said charge pattern by directing a second flow of electrons onto said areas of storage dielectric material and accelerating said second flow of electrons to a second potential to establish a second equilibrium potential of a value differing from that of said first equilibrium potential for the surfaces of said areas of storage dielectric material.

14. A method of operating a storage tube as claimed in claim 13, wherein the step of placing a charge pattern is performed :by applying an input signal to said areas of electrically conductive material while scanning the surface of said areas of storage dielectric material with said first ow of electrons.

15. A method of operating a storage tube as claimed in claim 13, wherein said step of placing a charge pattern is accomplished by maintaining substantially constant said first potential, varying the current density of said first flow of electrons in accordance with the desired input information, and scanning said' first flow of electrons over the surfaces of said areas of storage dielectric material.

16. A method of operating a storage tube as claimed in claim 13, wherein said step of deriving an output signal is performed by scanning the surface of Said areas of storage dielectric material with a constant density, second flow of electrons.

17. A method of operating a storage tube as claimed in claim 13, wherein said step of deriving an output signal is accomplished by scanning the surface of said areas of storage dielectric material with a constant density, second flow of electrons and collecting a return beam of secondary electrons emitted from said areas of storage dielectric material.

18. A method of operating a storage tube as claimed in claim 13, wherein the step of placing a charge pattern is accomplished by converting a radiation image into a corresponding electron image, and directing said electron image onto the surface of said areas of storage dielectric material.

References Cited UNITED STATES PATENTS 2,896,110 7/1959 Hansen 315-12 3,067,348 12/ 1962 Ochs 313-67 3,202,856 8/1965 Davis 313-89 3,379,914 4/1968 Koda et al. 313-89 RODNEY D. BENNETT, Jr., Primary Examiner J. P. MORRIS, Assistant Examiner 

